In welding lengths of pipe end to end, as in the welding together of lengths of pipe making up a pipe line, the abutting pipe ends must be matched as accurately as possible as to circularity and peripheral spacing. During welding, the pipes are not abutted together in contact, but a small uniform gap between the pipe ends is provided in order that welding may be accomplished through the entire wall thicknesses. In the case of relatively thick walled pipes, a great deal of force is required to urge the pipe ends to be as nearly as possible of circular form. Previous apparatuses known in the art have not been capable of creating forces of sufficiently large magnitude that the pipe ends may be made completely circular. In addition to the requirement for large forces at the clamps to make the pipe ends circular, it is also required of the pipe clamps that they be capable of holding the pipe ends accurately with the required small spacing or gap therebetween and so that the pipe ends do not become cocked one relative to the other during the welding operation. The pipe lengths are awkward to handle, being at least forty feet in length, and with the large wall thicknesses employed for the pressures now employed in pipelines, the weight of the pipe lengths is very great. Pipes are often welded into pipelines in multiple lengths; for example, the standard forty foot lengths of pipe may be welded together, or double jointed, to produce lengths of eighty feet, or may be triple jointed to produce lengths of one hundred and twenty feet. This is done so that the number of welds to be made in the field in assembling the pipeline may be reduced, the double jointing or triple jointing being done under controllable conditions where the pipe lengths may be more easily supported.
In welding pipe lengths together, it is necessary that a back up assembly be provided around the interiors of the pipe ends so that welding material will not be projected into the pipes, and so that the weld itself will not bulge into the pipe interior to form a discontinuity along the pipe wall. In order that good welds between the pipe ends may be achieved, the back up assembly must be continuous around the pipe periphery beneath the location of the weld, and must be firmly held in place against the interior pipe walls. The back up assembly must be positionable across the gap between the pipe ends after the pipes have been placed in position. Since the back up assembly must be retracted for movement to the location of the weld and must be expanded to provide adequate back up, previous apparatuses have almost invariably had peripheral gaps or irregularities whereby adequate back up for welds was not provided.
The present invention solves all or most of the above discussed problems.